About 7 p.m., the boys were at Corcoran Beach near 21st Avenue with an adult. One of the boys found a "gold shaped egg made of cardboard and a small fuse," Kent said.

They brought it back to a relative's home on the 500 block of Corcoran Avenue, where the 15-year-old lit the fuse, deputies said. The object exploded and launched shrapnel at the boy's torso. It also resulted in the "potential loss of his hand," Kent said.

Deputies and Central firefighters responded, and the boy was transported by helicopter to Stanford Hospital, authorities said.

Kent said it did not appear that the object was left on the beach to harm anyone, but deputies combed the beach and the surrounding area for other similar devices. They found none.

Deputies said anyone who finds similar objects should not touch them; instead call 911.

Kent said fireworks can be dangerous and can harm people.

"This tragedy is every parent's worst nightmare," said Sheriff-elect Jim Hart. "This is why we seize commercial-grade fireworks and step up enforcement on the Fourth of July."

The Sheriff's Office asks anyone with information to call 831-471-1121.